Incongruence in lighting between encoding and retrieval lowers performance in face memory tasks

The effect of uniform lighting on face identity processing is little understood, despite its potential influence on our ability to recognize faces. Here, we investigated how changes in uniform lighting level affected face identification performance during face memory tests. Observers (N=24) learned 16 faces in a randomized sequence (each face presented twice; stimulus duration: 2000 ms/presentation), followed by a memory test (16 studied faces and 16 distractor faces; stimulus duration: 350 ms) where observers judged whether the faces presented were studied before or novel. Face stimuli were presented under uniform bright or dim illuminations (mean luminance = 7.07 and 1.13 cd/m², within the photopic and mesopic ranges respectively). Lighting across the face learning (“encoding”) and the memory test (“retrieval”) sessions could be the same (“congruent”) or different (“incongruent”), resulting in four experimental conditions: (1) Bright/Dim (incongruent: encoding bright faces, retrieving dim faces); (2) Bright/Bright (congruent); (3) Dim/Bright (incongruent); and (4) Dim/Dim (congruent). Results showed that sensitivities (d’) to face identification were significantly above zero but varied across conditions (d’=0.885–1.698). In particular, incongruent lighting levels across sessions (Bright/Dim and Dim/Bright) significantly reduced d’ by 0.594 on average and introduced conservative biases (c=0.239–0.275) compared to congruent lighting levels (Bright/Bright and Dim/Dim), which had no significant bias. No significant differences in d’ were detected between the two congruent lighting conditions (Bright/Bright vs. Dim/Dim) and between the two incongruent lighting conditions (Bright/Dim vs. Dim/Bright). Thus, incongruent lighting deteriorated performance in face identification. These findings implied that the level of uniform lighting should be considered in an illumination-specific face representation and potential applications such as eyewitness testimony.